1. Field of the Invention
This invention relates generally to optical sensing of droplets in ink jet printers and more particularly to two-dimensional differential optical sensors for sensing the position of ink droplets relative thereto while they are in flight.
2. Description of the Prior Art
Generally, pagewidth ink jet devices of the continuous stream type employ a printhead having multiple nozzles from which continuous streams of ink droplets are emitted and directed to a recording medium or a collecting gutter. The printhead has an aperture plate with at least one row of nozzles or orifices through which the ink is ejected under pressure to form a row of parallel streams. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up in a series of uniform droplets at a fixed distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage by electrodes positioned adjacent the streams at the location where they break up into the droplets. The droplets which are charged are deflected by an electric field either into a gutter for ink collection and reuse, or to a specific location on the recording medium, such as paper, which may be continuously transported at a relatively high speed across the paths of the droplets.
Printing information is transferred to the droplets from each nozzle through charging by the electrodes, the charging control voltages are applied to the charging electrodes at the same frequency as that which the droplets are generated. This permits each droplet to be individually charged so that it may be positioned at a distinct location different from all other droplets or sent to the gutter. Printing information cannot be transferred to the droplets properly, unless each charging electrode is activated in proper phase with the droplet formation at the associated ink stream. As the ink droplets proceed in flight towards the recording medium, they are passed through a static electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium. Thus, to calibrate the ink jet printer so that the ink droplets impact the desired locations on the recording medium, the trajectories of the ink droplets for each nozzle must be determined and adjusted. Each nozzle is responsible for printing a line segment and the droplets emitted thereby may follow a plurality of trajectories in a deflection plane depending on its charge to print a series of adjacent pixels to produce a line segment on the recording medium.
U.S. Pat. No. 4,510,504 to Tamai et al discloses an ink droplet sensor for a multi-jet ink jet printer comprising a light emitter and a plurality of light receivers. In one embodiment, each nozzle for ejecting ink droplets has a light emitter that is to one side of the ink droplet flight path or trajectory and a set of light receivers on the other side of the droplet trajectory. The light refracted from a passing droplet to one of the receivers of the set determines, in combination with the intensity of the refracted light sensed, the flight path or trajectory of the droplet. In another embodiment, the light emitter and at least one of the plurality of receivers are aligned with the droplet trajectory of each nozzle. The light emitters and receivers are mounted in a common base plate and are substantially coplanar with each other.
U.S. Pat. No. 4,344,078 to Houston discloses a continuous stream ink jet printing system with an optical fiber sensor array positioned adjacent and upstream from a recording medium. A test gutter is positioned on the downstream side of the recording medium. The sensor array is used when the recording medium is not present to calibrate the charging voltages for a plurality of droplet streams. The sensor array includes two optical fiber sensors for each stream of droplets, one for each endmost droplet trajectory. Each sensor comprises an input fiber which directs a beam of light to two output fibers, identified as A and B fibers. Groups of the A and B fibers are terminated at common photodetectors requiring the A and B fibers to cross each other's paths. This is accomplished by fabricating the A fibers in one plane and the B fibers in a second adjacent plane.
U.S. Pat. No. 4,498,004 to Adolfsson et al discloses a measuring device having a transducer and an electronic unit interconnected by at least one optical fiber. The electronic unit has one or more light sources for transmitting light via the fibers to a sensor element, forming a part of the transducer, and one or more light detectors. At least one sensor element has a non-linear relationship between the incident light intensity illuminating the sensor element and the intensity of the light emitted from the sensor element. The light sources are arranged to emit light having at least two different light intensities and the detectors are arranged to measure the light coming from the sensor element.
U.S. Pat. No. 4,550,322 to Tamai discloses a drop sensor for an ink jet printer for determining the two-dimensional position of an ink drop as it flies through the sensor. A plurality of light emitting elements emit light beams which cross in pairs in the space defined by the sensor. A plurality of light receiving elements are aligned to receive the light beams projected from specific light emitting elements and to generate signals having magnitudes proportional to the intensities of the light received by the light receiving elements. A plurality of differential amplifier circuits compare the outputs of adjacent light receiving elements to determine whether an ink drop is coincident with selected matrix points within the sensor.
U.S. Pat. No. 4,551,731 to Lewis et al discloses a continuous stream type ink jet printer which has a detector means provided which sense values representative of droplet placement errors of test droplets in the direction of relative movement of the surface of the recording medium and the printer, and has control means responsive to the sensed values to advance or retard the application of the charge to the droplets to correct for the droplet placement errors.
U.S. Pat. No. 4,255,754 to Crean et al discloses the use of paired photodetectors to sense ink drops, one each for two output fibers that are used to generate an electrical zero crossing signal. The zero crossing signal is used to indicate alignment or misalignment of a droplet relative to the bisector of a distance between two output fibers. The sensor of this patent employs one input optical fiber and at least two output optical fibers. The free ends of the fibers are spaced a small distance from each other; the free end of the input fiber is on one side of the flight path of the droplets and the free end of the output fibers are on the opposite side. The remote end of the input fiber is coupled to a light source, such as an infra-red light emitting diode (LED). The remote ends of each output fiber are coupled to photodetectors such as, for example, a photodiode responsive to infra-red radiation. The ink is substantially a dye dissolved in water and is, of course, transparent to infra-red light, thus reducing the problems of contamination usually associated with ink droplet sensors. The photodiodes are coupled to differential amplifiers so that the output of the amplifiers are measurements of the location of droplets relative to the bisector of the distance between the output fiber ends confronting their associated input fibers and droplets passing therebetween. Amplifier outputs are used in servo loops to position subsequently generated droplets to the bisector location where equal amounts of light are blocked from each output fiber by passing droplets. The temporal zero crossing may be used, depending upon its orientation with respect to the droplet stream direction, as a time reference to measure the velocity of the drop. Therefore, the droplet velocity information may be used in a servo loop to achieve a desired velocity. The patent to Crean et al therefore discloses sensing the ink droplets in the plane of their travel and deflection.
Using an orthogonal coordinate system, the nominal trajectory of the uncharged droplets from the nozzle to the recording medium is the Z axis, and the deflection of the charged droplets by the deflection field is the X axis. The height of the droplet perpendicular to this XZ plane is the Y direction, and as disclosed in this patent, an ink droplet passing exactly through the bisector of the two output or receiving fibers cannot detect droplet position in the Y direction. This is because the sensor optical axis is perpendicular to the XZ plane, and the sensor differential output signal is not predictably affected by droplet Y position. By using one of these differential droplet sensors at a location midway between adjacent nozzles, the stitch point between end droplets can be controlled so that the segments of each line of droplets to be printed by each adjacent nozzle may be adjusted to prevent gaps or overprinting in the X direction on the recording medium. However, no sensing and control of the droplets Y position concurrently with the sensing and control of the droplets in the X direction is possible with this configuration.
U.S. Pat. No. 4,751,517 to Crean et al discloses a two-dimensional differential optical sensor for a continuous stream ink jet printer which senses the position of the droplets in the X or deflection direction and in the Y direction by combining a first single optical axis sensor of the type disclosed in U.S. 4,255,754 with another similar single optical axis sensor inclined at a predetermined angle with the first sensor, in order to monitor concurrently the droplet position in both the deflection direction and a direction perpendicular to it. A stitch point is the interface between printed line segments and was established at each droplet stream's deflection boundary (endmost droplet trajectory). The stitch point measurement is nominally perpendicular to the undeflected droplet trajectory and is made in a plane nominally parallel to the printing plane (i.e., recording medium's surface). A sensor according to U.S. 4,255,754 was located at each stitch point measurement location, and every other one was modified according to U.S. 4,751,517 wherein another set of optical sensors was incorporated, but with the optical axis at a nominally fixed angle .theta. with respect to the stitch point measurement optical axis in a second plane. Through calculations using the deflection voltages corresponding to droplet deflection sensed at the stitch and inclined sensor optical axes, the absolute droplet elevation was determined. The measured stitch and elevation (sagittal) values were stored in the controller of the printer and used for subsequent corrections of droplet trajectories.
Several problems remain with the approach of U.S. 4,751,517. First, the use of a two plane structure for the combined sensors resulted in fabrication problems involving accurate positioning of the stitch and sagittal light beam axes with respect to each other, both horizontally (deflection plane or X direction) and in the ink stream or Z direction. In practice, this has meant a time consuming characterization of the sensor structure, with individual parameters stored for each sagittal/stitch pair; viz., the absolute spatial equations for each pair of optical axes. Further, the inability to both hold the sagittal (elevation) to stitch axis separations in the Z direction constant and to characterize this factor results in an uncorrected measurement error in both the stitch and sagittal values optically obtained by the sensors, so that subsequent droplet placement errors occurred.
Another difficulty with this scheme of performing a sagittal measurement and calculation is encountered because they were done at only every other stitch point or droplet stream deflection boundary. Even if a perfect measurement of droplet elevation was made, there is no way to correct for the significant error introduced at the non-sagittally measured boundaries by variation in the deflection plane tilt (refer to FIG. 3, deflection plane 70 of U.S. 4,751,517) other than by acquiring a priori knowledge of the absolute value of the tilt angle of the droplet deflection plane for each droplet stream. Further, this structure is quite complex, requiring a total of three light sources and three differential receiver pairs per boundary of each set of droplet trajectories per nozzle. If a sagittal measurement were made at each boundary to avoid the problem of deflection plane tilt variation, then four light sources and four differential receiver pairs per boundary would be required. Finally, the use of a multiplicity of light sources and receiver pairs can create undesired spurious or secondary optical light beams which are erroneously sensed by the receiver pairs unless the light emission angles are kept very small or the light sources are independently switched off when not being used.